1. Field of the Invention
The present invention relates to an oxygen sensor device used for controlling an air-fuel ratio in an internal combustion engine such as of automobiles. More specifically, the invention relates to an oxygen sensor device incorporating a heater therein and is capable of detecting the concentration of oxygen within a very short period of time.
2. Description of the Prior Art
Modern internal combustion engines of automobiles are employing a method of detecting the concentration of oxygen in the exhaust gas, and controlling the amounts of the air and fuel supplied into the internal combustion engine based on the detected value, in order to decrease harmful substances such as CO, HC and NOX emitted from the internal combustion engine.
As a device for detecting the concentration of oxygen, there has heretofore been known a cylindrical oxygen sensor device of the indirectly heated type having a structure as shown in, for example, FIG. 18. The oxygen sensor device is constituted by a cylindrical tube 31 made of a solid electrolyte such as zirconia having oxygen ion conducting property with its one end being closed. On the inner surface of the cylindrical tube 31 is provided a reference electrode 32 that comes in contact with a reference gas such as the air. On the outer surface of the cylindrical tube 31 is formed a measuring electrode 33 that comes in contact with a gas to be measured such as exhaust gas. Further, on the surface of the measuring electrode 33 is formed various porous ceramic layers 34 depending upon the use of the oxygen sensor device.
In, for example, a so-called stoichiometric air-fuel sensor (xcex sensor) used for controlling the air-fuel ratio (A/F ratio) near 1, the porous ceramic layer 34 formed on the surface of the measuring electrode 33 works as a protection layer, and a difference in the oxygen concentration between the inner surface and the outer surface of the cylindrical tube 31 is detected at a predetermined temperature to control the A/F ratio in the engine intake system.
On the other hand, in a so-called wide-range air-fuel ratio sensor (AF sensor) used for controlling the A/F ratio over a wide range, the porous ceramic layer formed on the surface of the measuring electrode 33 contains fine pores and works as a gas diffusion rate-determining layer, and in which a voltage is applied to the cylindrical tube 31 of a solid electrolyte through a pair of electrodes 32 and 33, and a limit current that is obtained is measured to control the A/F ratio in a lean burn region.
In either the above-mentioned xcex sensor or AF sensor, the sensing portion (where the reference electrode 32 and the measuring electrode 33 are provided) must be heated up to an operation temperature of about 700xc2x0 C. For this purpose as shown in FIG. 18, a rod-like heater 35 is inserted in the inner space of the cylindrical tube 31 to heat the sensing portion up to the operation temperature (activating temperature).
In recent years, strict regulations have been enforced against the exhaust gases and, hence, it has been urged to detect CO, HC and NOx from immediately after the start of the engine. With the cylindrical oxygen sensor device of the indirectly heating type in which the above-mentioned heater 35 is inserted in the cylindrical tube 31, however, an extended period of time (activating time) is required before the sensing portion is heated up to the activating temperature leaving a problem in that regulations against the exhaust gases cannot be coped with to a sufficient degree.
To solve this problem, Japanese Unexamined Utility Model Publication (Kokai) No. 199666/1986 proposes a flat plate-type oxygen sensor device incorporating a heater as shown in FIG. 19. In this oxygen sensor device, space for a reference gas is formed in a solid electrolyte 39 of a flat plate, a measuring electrode 37 and a reference electrode 38 are formed on the outer surface and on the inner surface of a flat wall 36 of the solid electrolyte 39, and a heater 42 is integrally laminated on the solid electrolyte 39. The heater 42 is constituted by a ceramic insulating board 40 of a flat plate in which a heat-generating member 41 is buried.
Further, Japanese Unexamined Patent Publication (Kokai) No. 206380/1998 proposes a cylindrical oxygen sensor device incorporating a heater therein. In this oxygen sensor device like the oxygen sensor device of FIG. 18, a reference electrode and a measuring electrode are provided on the inner surface and on the outer surface of the cylindrical tube of a solid electrolyte, but having a gas-permeable porous insulating layer formed on the surface of the measuring electrode and a platinum heat-generating member provided in the insulating layer where the gas-permeability is low.
Unlike those of the conventional indirectly heated type, the above-mentioned flat-plate or cylindrical oxygen sensor incorporating a heater can be quickly heated owing to its direct-heating system, and the sensing portion can be quickly activated.
However, the oxygen sensor device incorporating a heater shown in FIG. 19 has poor durability and heat resistance due to its flat-plate shape, and is liable to be broken during the operation.
The oxygen sensor device incorporating a heater proposed in Japanese Unexamined Patent Publication (Kokai) No. 206380/1998 is manufactured by forming a cylindrical solid electrolytic portion by firing, forming the electrodes by plating or sputtering, and forming the insulating layer by plasma melt-injection method. In other words, this oxygen sensor device is produced by a complex method through an increased number of steps accompanied by such problems as poor yield and increased manufacturing cost. Besides, since a porous insulating layer is formed on the whole surface of the measuring electrode while burying the heat-generating member in the insulating layer, the junction strength of the heater portion is small lacking durability and heat resistance.
It is therefore an object of the present invention to provide an oxygen sensor device incorporating a heater therein, which can be activated within a short period of time, exhibits excellent durability and heat resistance, and can be easily produced.
According to the present invention, there is provided an oxygen sensor device comprising:
a cylindrical tube of a ceramic solid electrolyte having an oxygen ion conducting property and with its one end being closed;
a reference electrode formed on an inner surface of said cylindrical tube; and
a measuring electrode formed on an outer surface of said cylindrical tube at a position at least opposed to said reference electrode; wherein
a ceramic layer is formed on the outer surface of said cylindrical tube, said ceramic layer having an opening portion and incorporating a heat-generating member therein;
said opening portion in said ceramic layer is formed at such a position that said measuring electrode is at least partly exposed therein; and
said heat-generating member is buried in said ceramic layer at a position at least near said measuring electrode.
In the oxygen sensor device of the invention, the outer surface of the cylindrical tube of the solid electrolyte is covered with the ceramic layer incorporating a heat-generating member therein and, besides, the heat-generating member is arranged near the measuring electrode (e.g., around the opening portion that works as a sensing portion). According to the present invention, therefore, the sensing portion is very efficiently heated by the heat-generating member and is quickly heated making it possible to shorten the time (activating time) until the activating temperature is reached. Besides, even compared with the conventional flat-plate type oxygen sensor device incorporating a heater shown in FIG. 19, the oxygen sensor device of the invention greatly shortens the activating time and exhibits excellent sensor response, since the heat-generating member has been arranged near the sensing portion.
In the oxygen sensor device of the invention, further, the ceramic layer incorporating the heater has a cylindrical shape which is integral with the cylindrical tube, exhibiting a large strength against stress from any direction and building up less stress therein compared with the flat-plate type oxygen sensor device incorporating the heater, and exhibits excellent heat resistance and durability.
Further, the oxygen sensor device of the invention can be produced by co-firing the cylindrical tube of the solid electrolyte and the ceramic layer incorporating the heat-generating member. Therefore, the oxygen sensor device of the invention is produced at a very decreased cost and is excellent even from the standpoint of economy as compared with the conventional oxygen sensor device obtained by separately preparing the oxygen sensor and the heater, and inserting the heater in the oxygen sensor.
Upon forming a porous ceramic layer on the surface of the measuring electrode, the oxygen sensor device incorporating a heater of the invention can be used as a stoichiometric A/F ratio sensor (xcex sensor) or a wide-range A/F ratio sensor (AF sensor).